A mouse model for cystic biliary dysgenesis in autosomal recessive polycystic kidney disease (ARPKD)

Molecular and cellular pathogenesis of autosomal recessive polycystic kidney disease

Autosomal recessive polycystic kidney disease (ARPKD) is an inherited disease characterized by a malformation complex which includes cystically dilated tubules in the kidneys and ductal plate malformation in the liver. The disorder is observed primarily in infancy and childhood, being responsible for significant pediatric morbidity and mortality. All typical forms of ARPKD are caused by mutations in a single gene, PKHD1 (polycystic kidney and hepatic disease 1). This gene has a minimum of 86 exons, assembled into multiple differentially spliced transcripts and has its highest level of expression in kidney, pancreas and liver. Mutational analyses revealed that all patients with both mutations associated with truncation of the longest open reading frame-encoded protein displayed the severe phenotype. This product, polyductin, is a 4,074-amino acid protein expressed in the cytoplasm, plasma membrane and primary apical cilia, a structure that has been implicated in the pathogenesis of different polycystic kidney diseases. In fact, cholangiocytes isolated from an ARPKD rat model develop shorter and dysmorphic cilia, suggesting polyductin to be important for normal ciliary morphology. Polyductin seems also to participate in tubule morphogenesis and cell mitotic orientation along the tubular axis. The recent advances in the understanding of in vitro and animal models of polycystic kidney diseases have shed light on the molecular and cellular mechanisms of cyst formation and progression, allowing the initiation of therapeutic strategy designing and promising perspectives for ARPKD patients. It is notable that vasopressin V2 receptor antagonists can inhibit/halt the renal cystic disease progression in an orthologous rat model of human ARPKD.

Genetic interaction studies link autosomal dominant and recessive polycystic kidney disease in a common pathway

Polycystic kidney disease (PKD) describes a heterogeneous collection of disorders that differ significantly with respect to their etiology and clinical presentation. They share, however, abnormal tubular morphology as a common feature, leading to the hypothesis that their respective gene products may function cooperatively in a common pathway to maintain tubular integrity. To study the pathobiology of one major form of human PKD, we generated a mouse line with a floxed allele of Pkhd1, the orthologue of the gene mutated in human autosomal recessive PKD. Cre-mediated excision of exons 3-4 results in a probable hypomorphic allele. Pkhd1(del3-4/del3-4) developed a range of phenotypes that recapitulate key features of the human disease. Like in humans, abnormalities of the biliary tract were an invariant finding. Most mice 6 months or older also developed renal cysts. Subsets of animals presented with either perinatal respiratory failure or exhibited growth retardation that was not due to the renal disease. We then tested for genetic interaction between Pkhd1 and Pkd1, the mouse orthologue of the gene most commonly linked to human autosomal dominant PKD. Pkd1(+/-); Pkhd1(del3-4/del3-4) mice had markedly more severe disease than Pkd1(+/+); Pkhd1(del3-4/del3-4) littermates. These studies are the first to show genetic interaction between the major loci responsible for human renal cystic disease in a common PKD pathway.

A mouse model of autosomal recessive polycystic kidney disease with biliary duct and proximal tubule dilatation

Autosomal recessive polycystic kidney disease (ARPKD) is caused by mutations in the polycystic kidney and hepatic disease (PKHD1) gene encoding the protein fibrocystin/polyductin. The aim of our study was to produce a mouse model of ARPKD in which there was no functional fibrocystin/polyductin to study the pathophysiology of cystic and fibrocystic disease in renal and non-renal tissues. Exon 2 of the gene was deleted and replaced with a neomycin resistance cassette flanked by loxP sites, which could be subsequently removed by Cre-lox recombinase. Homozygous Pkhd1(del2/del2) mice were viable, fertile and exhibited hepatic, pancreatic, and renal abnormalities. The biliary phenotype displayed progressive bile duct dilatation, resulting in grossly cystic and fibrotic livers in all animals. The primary cilia in the bile ducts of these mutant mice had structural abnormalities and were significantly shorter than those of wild-type (WT) animals. The Pkhd1(del2/del2) mice often developed pancreatic cysts and some exhibited gross pancreatic enlargement. In the kidneys of affected female mice, there was tubular dilatation of the S3 segment of the proximal tubule (PT) starting at about 9 months of age, whereas male mice had normal kidneys up to 18 months of age. Inbreeding the mutation onto BALBc/J or C57BL/6J background mice resulted in females developing PT dilatation by 3 months of age. These inbred mice will be useful resources for studying the mechanisms underlying the pathogenesis of ARPKD.

Fibrocystin/polyductin, found in the same protein complex with polycystin-2, regulates calcium responses in kidney epithelia

Recent evidence suggests that fibrocystin/polyductin (FPC), polycystin-1 (PC1), and polycystin-2 (PC2) are all localized at the plasma membrane and the primary cilium, where PC1 and PC2 contribute to fluid flow sensation and may function in the same mechanotransduction pathways. To further define the exact subcellular localization of FPC, the protein product encoded by the PKHD1 gene responsible for autosomal recessive polycystic kidney disease (PKD) in humans, and whether FPC has direct and/or indirect cross talk with PC2, which, in turn, is pivotal for the pathogenesis of autosomal dominant PKD, we performed double immunostaining and coimmunoprecipitation as well as a microfluorimetry study of kidney tubular epithelial cells. FPC and PC2 are found to completely or partially colocalize at the plasma membrane and the primary cilium and can be reciprocally coimmunoprecipitated. Although incomplete removal of FPC by small interfering RNA and antibody 803 to intracellular epitopes of FPC did not abolish flow-induced intracellular calcium responses, antibody 804 to extracellular epitopes of FPC blocked cellular calcium responses to flow stimulation. These findings suggest that FPC and polycystins share, at least in part, a common mechanotransduction pathway.

Molecular Mechanism of Liver Development and Regeneration

The liver is the central organ for metabolism and has strong regenerative capability. Although the liver has been studied mostly biochemically and histopathologically, genetic studies using gene-targeting technology have identified a number of cytokines, intracellular signaling molecules, and transcription factors involved in liver development and regeneration. In addition, various in vitro systems such as fetal liver explant culture and primary culture of fetal liver cells have been established, and the combination of genetic and in vitro studies has accelerated investigation of liver development. Identification of the cell-surface molecules of liver progenitors has made it possible to identify and isolate liver progenitors, making the liver a unique model for stem cell biology. In this review, we summarize progresses in understanding liver development and regeneration.

Inhibition of intrahepatic bile duct dilation of the polycystic kidney rat with a novel tyrosine kinase inhibitor gefitinib

The polycystic kidney (PCK) rat represents a liver and kidney cyst pathology corresponding to Caroli's disease with congenital hepatic fibrosis and autosomal recessive polycystic kidney disease. We previously reported that an epidermal growth factor receptor tyrosine kinase inhibitor, gefitinib (Iressa), significantly inhibited the abnormal growth of biliary epithelial cells of PCK rats in vitro. This study investigated the effects of gefitinib on cyst pathogenesis of the PCK rat both in vitro and in vivo. A three-dimensional culture model of biliary epithelial cells in the collagen gel matrix was used for in vitro analysis. For in vivo experiments, PCK and control rats were treated with gefitinib between 3 and 10 weeks of age. In vitro, gefitinib had strong inhibitory effects on biliary cyst formation of PCK rats. In vivo, treatment with gefitinib significantly inhibited the cystic dilatation of the intrahepatic bile ducts of PCK rats, which was accompanied by improvement of liver fibrosis. By contrast, no beneficial effects were observed on renal cyst development because of the treatment. These results suggest that signaling pathways mediated by epidermal growth factor receptor are involved in biliary dysgenesis of the PCK rat, with the mechanisms of cyst progression being different between the liver and kidney.

Diagnosis, pathogenesis, and treatment prospects in cystic kidney disease

Cystic kidney diseases (CKDs) are a clinically and genetically heterogeneous group of disorders characterized by progressive fibrocystic renal and hepatobiliary changes. Recent findings have proven the cystogenic process to be compatible with cellular dedifferentiation, i. e. increased apoptosis and proliferation rates, altered protein sorting and secretory characteristics, as well as disorganization of the extracellular matrix. Compelling evidence suggests that cilia play a central pathogenic role and most cystic kidney disorders converge into a common pathogenic pathway. Recently, several promising trials have further extended our understanding of the pathophysiology of CKD and may have the potential for rational personalized therapies in future years. This review aims to summarize the current state of knowledge of the structure and function of proteins underlying polycystic kidney disease, to explore the clinical consequences of changes in respective genes, and to discuss potential therapeutic approaches.

Development and characterization of a cholangiocyte cell line from the PCK rat, an animal model of Autosomal Recessive Polycystic Kidney Disease

In the PCK rat, a rodent model of Autosomal Recessive Polycystic Kidney Disease (ARPKD), a spontaneous splicing mutation of Pkhd1 initiates hepatic cyst development. Cystic cholangiocytes possess short and malformed cilia that do not express fibrocystin, the Pkhd1 protein. During the disease course, cysts continue to grow; however, the mechanisms underlying cyst progression are unclear due in part to the lack of suitable cell lines to study cystogenesis. Here, we describe the development of a PCK-derived cholangiocyte cell line (PCK-CCL). Normal rat cholangiocytes (NRCs) were used as a control. The PCK-CCL maintained a cholangiocyte phenotype as assessed by the expression of the CK-19, CK-7 and GGT. PCK-CCL grown on collagen formed a polarized monolayer with well-developed junctional complexes, and distinct apical and basolateral membranes. Compared to NRCs, cilia in the PCK-CCL were short and malformed and did not express fibrocystin. The PCK-CCL exhibited a higher rate of proliferation (P<0.05) with a doubling time approximately half that of NRCs. By RT-PCR analysis of exons 33-37, an approximately 800 bp product of Pkhd1 was amplified in NRCs. In contrast and as expected, in the PCK-CCL, the Pkhd1 amplicon was smaller ( approximately 630 bp) reflecting the IVS35-2A --> T mutation. PCK-CCL and NRCs seeded in 3-D cultures formed cystic structures; however, the PCK cysts expanded progressively up to day 21 while cysts formed by NRCs remained the same size after day 9. In summary, we have developed a cholangiocyte cell line from the PCK rat that retains properties of the cholangiocytes lining hepatic cysts in vivo. The cells have been grown continuously for approximately 18 month and 45 passages without crisis or senescence. The morphology and growth characteristics of the PCK-CCL are consistent with those seen in vivo in the PCK rat, suggesting that this cell line will be useful in dissecting the mechanisms of hepatic cyst formation.

Is congenital hepatic fibrosis a pure liver disease?

OBJECTIVES

An association between congenital hepatic fibrosis (CHF) and several different conditions is being increasingly recognized. We aimed to investigate, prospectively, these associated disorders and the clinical consequences for patients with CHF.

MATERIALS AND METHODS

CHF was diagnosed using liver biopsy, abdominal ultrasound (US), Doppler US, upper endoscopy, and abdominal computed tomography (CT) in 19 patients (13 women, 6 men). CT portography and splenoportography with digital subtraction angiography were performed if indicated. Endoscopic retrograde cholangiopancreatography (ERCP) was performed to investigate the extent of portal vein involvement of the common bile duct if it existed, to remove a stone located in the common bile duct when documented, and to confirm the diagnosis of Caroli's syndrome. Cranial MRI was done when clinical findings suggested brain involvement.

RESULTS

The mean age of the patients was 29.47+/-12.06, ranging from 13 to 57. CHF-associated diseases were Caroli's syndrome, polycystic kidney disease, cavernous transformation of the portal vein, Joubert's syndrome, von Meyenburg complex, polydactyly, medullary sponge kidney, and pancreatic duct atrophy. In two cases, cholangiocarcinoma had developed. There was only one case with pure CHF. Portosystemic shunt, TIPS, or splenectomy were performed in some cases to control bleeding from esophageal varices. Papillotomy and stone extraction from the common bile duct were performed in four patients with Caroli's syndrome complicated by cholangitis. Three patients died of complications of CHF. Two patients with Caroli's syndrome underwent liver transplantation.

CONCLUSION

In this prospective study, it seems that CHF is not a pure liver disease but rather a multiorgan disorder involving the brain, portal vein, kidneys, and bile ducts. In most cases, the clinical picture includes other organ involvement, rather than purely the liver parenchyma.

Cholangiocyte biology

PURPOSE OF REVIEW

Cholangiocytes are increasingly recognized as biologically important epithelia because of the diverse array of cellular processes in which they participate. Collectively, these processes define normal function and, when disturbed, account for abnormalities that cause disease. Advances in animal models and sophisticated technology in imaging and gene silencing have led to substantial progress in defining the roles that cholangiocytes play in signaling, transport of water, ions and solutes, and alterations that result in cholestasis. The pace of advances in technology justifies a yearly summary to identify trends, and inform the readership of the most significant developments in cholangiocyte biology.

RECENT FINDINGS

The main areas of recent progress include insights into the molecular mechanisms of bile secretion and the development of new experimental models and technologies. Major advances have also included the identification of novel roles for receptors and better understanding of mechanistic pathways and biologic processes.

SUMMARY

Understanding the key mechanistic and biologic processes in cholangiocytes is required to generate hypotheses and therapies relevant to disease. This compendium of current activities in cholangiocyte biology may promote collegial sharing and exchange of novel concepts, ideas, reagents and probes, thereby promoting positive advances in the field.

Clinical and molecular characterization defines a broadened spectrum of autosomal recessive polycystic kidney disease (ARPKD)

The autosomal recessive form of polycystic kidney disease (ARPKD) is generally considered an infantile disorder with the typical presentation of greatly enlarged echogenic kidneys detected in utero or within the neonatal period, often resulting in neonatal demise. However, there is an increasing realization that survivors often thrive into adulthood with complications of the ductal plate malformation, manifesting as congenital hepatic fibrosis and Caroli disease, becoming prominent. Previous natural history studies have concentrated almost exclusively on the infantile presenting group. However, developments in understanding the genetic basis of ARPKD, through identification of the disease gene, PKHD1, have allowed exploration of the etiology in patients with ARPKD-like disease or congenital hepatic fibrosis presenting later in childhood or as adults. In the current study we retrospectively reviewed the clinical records, and where possible performed PKHD1 mutation screening, in patients diagnosed with ARPKD or congenital hepatic fibrosis at the Mayo Clinic, Rochester, MN, from 1961 to 2004. Of a total of 133 cases reviewed, 65 were considered to meet the diagnostic criteria with an average duration of follow-up of 8.6 +/- 6.4 years. Fifty-five cases had ARPKD and 10 had isolated congenital hepatic fibrosis with no or minimal renal involvement. The patients were analyzed as 3 groups categorized by the age at diagnosis; <1 years (n = 22), 1-20 years (n = 23), and >20 years (n = 20). The presenting feature in the neonates was typically associated with renal enlargement, but in the older groups, more often involved manifestations of liver disease, including hepatosplenomegaly, hypersplenism, variceal bleeding, and cholangitis. During follow-up, 22 patients had renal insufficiency and 8 developed end-stage renal disease (ESRD), most from the neonatal group. Liver disease was evident on follow-up in all diagnostic groups but particularly prevalent in those diagnosed later in life. A total of 12 patients died, 6 in the neonatal period, but 86% of patients were alive at 40 years of age. The likelihood of being alive without ESRD differed significantly between the diagnostic groups with 36%, 80%, and 88% survival in the 3 diagnostic groups, respectively, 20 years after the diagnosis. Considerable evidence of intrafamilial phenotype variability was observed. Mutation analysis was performed in 31 families and at least 1 mutation was detected in 25 (81%), with 76% of mutant alleles detected in those cases. Consistent with the relatively mild disease manifestations in this population, the majority of changes were missense (79%) and no case had 2 truncating changes. Mutations were detected in all diagnostic groups, indicating that congenital hepatic fibrosis with minimal kidney involvement can result from PKHD1 mutation. The finding of 6 cases with no detected mutations may represent missed mutations or possible evidence of genetic heterogeneity. The current study indicates a broadened spectrum for the ARPKD phenotype and that later presenting cases with predominant liver disease should be considered part of ARPKD.

Inhibition of Pkhd1 impairs tubulomorphogenesis of cultured IMCD cells

Fibrocystin/polyductin (FPC), the gene product of PKHD1, is responsible for autosomal recessive polycystic kidney disease (ARPKD). This disease is characterized by symmetrically large kidneys with ectasia of collecting ducts. In the kidney, FPC predominantly localizes to the apical domain of tubule cells, where it associates with the basal bodies/primary cilia; however, the functional role of this protein is still unknown. In this study, we established stable IMCD (mouse inner medullary collecting duct) cell lines, in which FPC was silenced by short hairpin RNA inhibition (shRNA). We showed that inhibition of FPC disrupted tubulomorphogenesis of IMCD cells grown in three-dimensional cultures. Pkhd1-silenced cells developed abnormalities in cell-cell contact, actin cytoskeleton organization, cell-ECM interactions, cell proliferation, and apoptosis, which may be mediated by dysregulation of extracellular-regulated kinase (ERK) and focal adhesion kinase (FAK) signaling. These alterations in cell function in vitro may explain the characteristics of ARPKD phenotypes in vivo.